HVAC: Space Heaters

Space heaters are typically used either to supplement an inadequate existing heating system or to save energy and defray energy costs in one of the situations described above. In either of these cases, look for a heater that has a design and safety features that match your needs. To pick a design that works well in a certain situation, it is important to understand the different types of space heaters that are available.

Radiant heaters. This type of heater provides warmth in the same way that the sun's rays or a warm fireplace does. Radiant heaters use a variety of heating elements (including quartz tubes, carbon tubes, metal coils, and halogen lamps) to emit radiated heat in a specific direction (Figure 1, next page). Some types use reflectors to focus the heat; others simply allow the heat to dissipate in the direction the heater is facing. Radiant heaters are better suited to heating objects (usually people) than entire spaces, and they work best in areas with open architecture (not many doors or other obstructions). Objects must be within the heater's line of sight to feel the heat. Over time, radiant heaters will increase air temperature as the people and objects that absorb the radiant heat directly transfer some of it to the air.

If you plan on being in a room for only a short period, a radiant heater is the best option for space heating. They can be more efficient over a short period than other heater types because radiant heaters avoid the energy needed to heat the entire room by instead directly warming the occupant of the room and the occupant's immediate surroundings. Radiant heaters are sometimes referred to as quartz, infrared, parabolic, ceramic, halogen, and reflective heaters.

Convection heaters. The main difference between a convection heater and a radiant heater is air flow. Convection heaters rely on the the natural pattern of rising hot air to heat a room. They are designed to heat an entire room by heating the air, rather than focusing the heat on any particular object, and so they work best in rooms that can be sealed off (usually by closing a door). One example is a portable electric baseboard-style space heater. With a baseboard heater, air circulates through the room and enters the baseboard heater on the floor. The air is then warmed by passing across the electric element in the heater and rises through the room. The cold air in the room descends to enter the heater, and the cycle is repeated. The most efficient convection heaters are oil- and water-filled heaters (Figure 2). These employ a heating element in a bath of oil or water. The oil or water remains heated while the unit is on and heats the surrounding air by way of natural air movement through the unit, similar to an electric baseboard heater. These oil- or water-filled units look like a portable radiator.

Combination heaters. Many space heaters employ both radiant and convection heating techniques. These combination heaters are sometimes regarded as having more flexible operating use requirements than conventional heaters. One example of this type of heater is a ceramic radiant heater that uses an internal fan to distribute the heat it generates (Figure 3). Combination heaters heat specific objects in addition to the air surrounding them, although they don't do either as well as a heater designed solely with either radiant or convection technology would. Combination heaters are also called heater fans.

Unit heaters. One type of space heater that dominates use in the commercial and industrial sectors is the unit heater, which is basically a forced-air furnace without ducts (Figure 4). It is a self-contained, permanently surface-mounted heater that can operate on either electricity or natural gas, though natural gas is the more common fuel used. As with all gas-fired combustion equipment, unit heaters must be vented to the outdoors to exhaust the combustion gas. These units require some installation work, but they can also provide a much greater amount of heating for a space than a typical portable space heater. Unit heaters are good for heating an add-on room, workshop, or garage without needing to add ductwork. They are most commonly used in warehouses, car dealerships, auto service bays, and other large open buildings. Even with their ability to create a large amount of heat for open areas, they are sometimes criticized for creating large temperature differences in a space, as well as for the noise they produce.

Utility heaters. These are used mainly at outdoor worksites and in other areas where spot heating is either helpful or necessary. They are generally oil- or propane-fueled, and they use a cylindrical shape to direct heating in a specific direction (Figure 5). They can be used in auto bays and other industrial areas that are often open to the elements. They are effective for outdoor worksite use where electrical outlets may not be convenient, although some units come fitted with an electric ignition that requires an outlet.

Common Features

• Not all space heaters have all of the features listed here; this is a list of the most commonly found and most highly desired space heater characteristics.

• • Adjustable heating element (providing multiple heat settings)
  • Adjustable fan
  • Carrying handle
  • Cool-touch exterior
  • Thermostatic control
  • Digital thermostat
  • Night light
  • Occupancy sensor
  • Oscillating unit
  • Overheat safety shut-off
  • Power indicator
  • Remote control
  • Silent operation
  • Timer
  • Tip-over safety alarm
  • Tip-over safety shut-off
  • Wheels

Once you've decided that space heaters are right for your business, it is helpful to answer a few questions to help determine the proper type of space heater that will suit your needs. For example:

• Do you have a preference on whether it uses electricity or natural gas? Are both available where the heat is needed?
• Where do you plan to use the space heater? In what areas or rooms, specifically? What tasks will be performed there?
• In general, how often and for how long will the heater be in use—periodically or constantly?
• What features are important for your application?

Sporadic usage, spot heating. If you need help "filling the gap" or simply adding some extra warmth while performing a task at home or at work, a radiant heater will be the best match. They provide heat almost instantaneously but must be located fairly close to warm an occupant effectively. The main caveat with these types of heaters is that they can be a fire hazard, so safety precautions must be taken in order to prevent fires, burns, or other damage. Examples of where these heaters might be used are for a person working at a desk or sitting on a couch; these perform well when the object to be warmed is relatively stationary.

In some instances, it makes sense to use a radiant heating panel: These are varying-sized electric panels that are mounted to a surface (wall, ceiling, or floor) and radiate heat to an object directly in front of them. They operate completely silently, are cool to the touch, and typically consume less power than a traditional radiant heater; they also produce less heat, but can work well to provide some heat for a person sitting at a desk or other small enclosed area with a nearby mounting surface.

Constant operation, whole-room heating. If you plan to use the space heater for extended periods of time or to heat an entire room and a conventional central heating system is impractical, a convection heater will most likely be your best choice. Either an oil- or water-filled radiator heater or a portable baseboard-style heater will work. Unlike radiant heaters, convection heaters cannot provide instantaneous spot heating but instead gradually warm the air in the room. They provide an even, comfortable heat through natural convection of the room air. Most of these heaters are silent and cool to the touch.

Varied operation, versatile tasks. If the space heater will be used for a variety of tasks, a combination heater—employing both radiant and convection techniques—is best. These are currently the most commonly used space heaters. Any space heater with a heating element and an internal fan can be classified as a combination heater. These heaters can be used to warm up an entire room (although not as evenly as a true convection heater) and to heat a stationary person (again, not as efficiently as a true radiant heater). Many people enjoy the versatility of these portable types. These heaters can also be a fire hazard, so it is important to operate them in areas where they're unlikely to come into contact with flammable materials such as paper.

Large open commercial or industrial spaces. Larger spaces typically benefit the most from installing a unit heater. Though their current typical efficiencies range between 55 and 80 percent, large Btu capacity and ductwork-free operation make the unit heater an attractive option in larger open spaces where a conventional forced-air system would be more costly. Because the Energy Policy Act of 2005 (EPAct 2005) includes new standards for unit heaters, their efficiencies will rise, offering an even greater potential for savings. Federal regulations that go into effect in 2008 will require all unit heaters to adopt features that only the most efficient models have now.

A typical portable electric space heater uses between 600 and 1,500 watts (W), with the medium heat setting consuming around 1,000 W (1 kilowatt). A typical application might use a space heater for 8 hours a day, 5 days a week. Assuming an electric rate of $0.09 per kilowatt-hour, a portable electric space heater used this way will cost approximately $15.26 to operate for one month. A radiant heating panel on the medium heat setting would cost about $9.16 per month. Operating cost noticeably increases for combustion heaters, with unit heaters weighing in at just under $400 each month for a midrange capacity and utility heaters costing a little over $300 each month for the midrange capacity (Table 1).

In general, the efficiency of an electric space heater is essentially 100 percent. In other words, all of the electricity it consumes will be given off as heat. Space heaters that burn natural gas have a typical efficiency ranging between 55 and 80 percent, because some of the heating energy in the fuel is lost during combustion. However, EPAct 2005 requires that all unit heaters manufactured in 2008 and beyond must have specific technologies that will help increase their average efficiency to around 80 percent.

Using Space Heaters to Save Energy

• Some advertisements for radiant space heaters claim that they save large quantities of energy. Although that can be true, it is only if they are used in a very specific manner. To achieve such a high amount of savings, you have to turn down the central heating system considerably and then use radiant space heaters to heat only those areas that are occupied by people. The radiant heat warms occupants and the objects surrounding them. Due to this direct radiant warming, the indoor air temperature does not need to be set as high, which reduces the amount of energy consumed by the central heating system.

Before you jump onto the space heater bandwagon, it will likely be beneficial to perform some easy and inexpensive energy-saving measures at your workplace. Any one of these could solve your heating problems without any additional heating equipment.

• Weatherize the building: Install caulking and weather stripping around doors and windows and add insulation.
• Clean or replace the furnace air filter regularly.
• Insulate heating ducts.
• Verify that heat registers are not blocked (by furniture or other objects) or clogged.
• Seal off registers in unused spaces.

These measures can be implemented in just a few hours, and any costs are usually recouped in savings over just a few months.

Space heater technology and features advance at a slow rate, so don't expect huge changes any time soon. However, EPAct 2005 does include standards for unit heaters; in particular, all unit heaters manufactured after July 2008 must:

• Be equipped with an intermittent ignition device
• Have power venting or an automatic flue damper

These requirements will increase the average efficiency of all new unit heaters to approximately 80 percent.